Cellular Respiration Part V: Oxidative Phosphorylation

Oxidative phosphorylation: electron transport and chemiosmosis Figure 9.16 Electron shuttles span membrane MITOCHONDRION 2 NADH or 2 FADH2 2 NADH 2 NADH 6 NADH 2 FADH2 Oxidative phosphorylation: electron transport and chemiosmosis Glycolysis Pyruvate oxidation Citric acid cycle Glucose 2 Pyruvate 2 Acetyl CoA What was oxygen’s role in cellular respiration?  2 ATP  2 ATP  about 26 or 28 ATP About 30 or 32 ATP Maximum per glucose: CYTOSOL

Without Oxygen… Most cellular respiration requires O2 to produce ATP Without O2, the electron transport chain will cease to operate In that case, glycolysis couples with fermentation or anaerobic respiration to produce ATP Emphasize that last bullet! Students think only in terms of “oxygen-based” respiration. They may not even be aware that respiration is possible anaerobically!

Anaerobic respiration uses an electron transport chain with a final electron acceptor other than O2, for example sulfate Fermentation uses substrate-level phosphorylation instead of an electron transport chain to generate ATP Also emphasize that anaerobic respiration simply occurs when a different “chemical” accepts the electrons instead of oxygen. Many substances can function as final electron acceptors. These include sulfate (SO42-), nitrate (NO3-), sulfur (S), or fumarate. Often organisms that utilize the molecules are obligate anaerobes.

Types of Fermentation Fermentation consists of glycolysis plus reactions that regenerate NAD+, which can be reused by glycolysis Two common types are alcohol fermentation and lactic acid fermentation Emphasize to student that there are many, many types of alcohols and not all are safe for human consumption!

Alcoholic Fermentation In alcohol fermentation, pyruvate is converted to ethanol in two steps, with the first releasing CO2 Alcohol fermentation by yeast is used in brewing, winemaking, and baking We are indeed referring to ethanol when we say “alcohol fermentation”.

i i   Figure 9.17 2 ATP  2 ATP Glucose Glycolysis Glucose 2 ADP  2 P i 2 ATP 2 ADP  2 P i 2 ATP Glucose Glycolysis Glucose Glycolysis 2 Pyruvate 2 NAD  2 NADH 2 CO2 2 NAD  2 NADH  2 H  2 H 2 Pyruvate Figure 9.17 Fermentation. Emphasize the similarities and differences between two different types of fermentation. Animations of each follow. 2 Ethanol 2 Acetaldehyde 2 Lactate (a) Alcohol fermentation (b) Lactic acid fermentation

  2 ADP  2 P i 2 ATP Glucose Glycolysis 2 Pyruvate 2 NAD 2 NADH Figure 9.17a 2 ADP  2 P i 2 ATP Glucose Glycolysis 2 Pyruvate 2 NAD  2 NADH 2 CO2  2 H  Note end product is ethanol and NAD is regenerated 2 Acetaldehyde 2 Ethanol (a) Alcohol fermentation

Alcoholic Fermentation

  2 ADP  2 P i 2 ATP Glucose Glycolysis 2 NAD 2 NADH  2 H Figure 9.17b 2 ADP  2 P i 2 ATP Glucose Glycolysis 2 NAD  2 NADH  2 H  2 Pyruvate Figure 9.17 Fermentation. 2 Lactate (b) Lactic acid fermentation

Lactic Acid Fermentation In lactic acid fermentation, pyruvate is reduced to NADH, forming lactate as an end product, with no release of CO2 Lactic acid fermentation by some fungi and bacteria is used to make cheese and yogurt Human muscle cells use lactic acid fermentation to generate ATP when O2 is scarce

Lactate Fermentation

Fermentation compared to Anaerobic & Aerobic Respiration All use glycolysis (net ATP = 2) to oxidize glucose and harvest chemical energy of food In all three, NAD+ is the oxidizing agent that accepts electrons during glycolysis The processes have different final electron acceptors: an organic molecule (such as pyruvate or acetaldehyde) in fermentation and O2 in cellular respiration Cellular respiration produces 32 ATP per glucose molecule; fermentation produces 2 ATP per glucose molecule

Anaerobic Respiration Obligate anaerobes carry out fermentation or anaerobic respiration and cannot survive in the presence of O2 Yeast and many bacteria are facultative anaerobes, meaning that they can survive using either fermentation or cellular respiration In a facultative anaerobe, pyruvate is a fork in the metabolic road that leads to two alternative catabolic routes

Ethanol, lactate, or other products Figure 9.18 Glucose Glycolysis CYTOSOL Pyruvate No O2 present: Fermentation O2 present: Aerobic cellular respiration MITOCHONDRION Ethanol, lactate, or other products Acetyl CoA Pyruvate as a key juncture in catabolism. Citric acid cycle

Evolutionary Significance of Glycolysis Ancient prokaryotes are thought to have used glycolysis long before there was oxygen in the atmosphere Very little O2 was available in the atmosphere until about 2.7 billion years ago, so early prokaryotes likely used only glycolysis to generate ATP Glycolysis is a very ancient process Never pass up an opportunity to connect a unit back to evolution!

But I eat more than sugar.. Catabolic pathways funnel electrons from many kinds of organic molecules into cellular respiration Glycolysis accepts a wide range of carbohydrates Proteins must be digested to amino acids; amino groups can feed glycolysis or the citric acid cycle Emphasize the literal meaning of “carbohydrate”, carbon-waters which have a generic formula of CH2O. Multiply that by 6 and you have glucose! Add bunches of those together through bunches of dehydration synthesis reactions and you have a starch.

Oxidative phosphorylation Figure 9.19 Proteins Carbohydrates Fats Amino acids Sugars Glycerol Fatty acids Glycolysis Glucose Glyceraldehyde 3- P NH3 Pyruvate Acetyl CoA Figure 9.19 The catabolism of various molecules from food. Citric acid cycle Oxidative phosphorylation

Regulation of Respiration Feedback inhibition is the most common mechanism for control If ATP concentration begins to drop, respiration speeds up; when there is plenty of ATP, respiration slows down Control of catabolism is based mainly on regulating the activity of enzymes at strategic points in the catabolic pathway Emphasize one last time that METABOLISM is the sum of CATABOLISM (breaking down or deconstructing) + ANABOLISM (synthesizing or building up)

It sounds simple but…… Why do you have to eat? What does it have to do with entropy? Why do you have to breathe? Can you give an AP Biology level answer to these questions?

Created by: Debra Richards Coordinator of Secondary Science Programs Bryan ISD Bryan, TX